The magnetic properties of FINEMET-type nanocrystalline alloys and isolated ferromagnetic AgCo nanoparticles are investigated both experimentally and numerically. Theoretical models of spins that simulate ideal nanocrystalline alloys and isolated nanoparticles are considered while their magnetic properties are derived from Monte Carlo simulation of low-temperature spin ordering. Interesting features such as magnetic polarization of the matrix due to penetrating fields arising from nanograins and the role played by the crystalline fraction in the overall L magnetic behaviour, in the case of nanocrystalline alloys are investigated. For isolated nanoparticles it is shown that the competition between surface and bulk anisotropy gives rise to surface spin disorder that, together with finite-size effects, is responsible for the experimentally observed lack of saturation of the magnetization in high applied fields. These simulation results are confirmed by experimental data obtained on FINEMET nanocrystalline alloys and isolated ferromagnetic AgCo colloidal nanoparticles.